1. Field of the Invention
The present invention relates to an automatic gain control apparatus and the related gain control method, and more particularly to a low cost automatic gain control apparatus.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a conventional radio signal receiving module 100, such as a digital television receiver. The radio signal receiving module 100 comprises an antenna 102, a radio frequency amplifier 104, a mixer 106, a SAW (Surface Acoustic Wave) filter 108, an intermediate frequency amplifier 110, an ADC (Analog-to-digital converter) 112, and a AGC (Automatic gain control) apparatus 114. The radio signal receiving module 100 is utilized to receive a radio signal Sr and output a corresponding digital signal Sd to a demodulator 116. The AGC apparatus 114 is utilized for detecting a power of the digital signal Sd and for adjusting a gain of the radio frequency amplifier 104 and the intermediate frequency amplifier 110. Since the configuration of the radio signal receiving module 100 is a feedback control configuration, the stability issue can be a critical problem of the radio signal receiving module 100. In other words, the radio signal receiving module 100 should not be influenced by the channel noise. In addition, the co-operation between the radio frequency amplifier 104 and the intermediate frequency amplifier 110 should be inter-influenced to further stabilize the radio signal receiving module 100. When the power of the radio signal Sr is varied, the AGC apparatus 114 adjusts the gain of the radio frequency amplifier 104 in order to keep the output of the radio frequency amplifier 104 falling within an appropriate range, as well as the intermediate frequency amplifier 110. Therefore, the reaction time of the AGC apparatus 114 to adjust the gains of the radio frequency amplifier 104 and the intermediate frequency amplifier 110 is another critical problem of the radio signal receiving module 100. Meanwhile, the cost, i.e., the device size, of the radio signal receiving module 100 is also a main consideration in designing the radio signal receiving module 100.
Conventionally, the AGC apparatus 114 of the radio signal receiving module 100 adjusts the gain of the radio frequency amplifier 104 and the intermediate frequency amplifier 110 by a digital signal power detector, such as a peak detector, which may occupy a large area. Furthermore, the AGC apparatus 114 may have a non-obvious take-over-point (TOP) to result in destabilizing the radio signal receiving module 100. In other words, the AGC apparatus 114 may adjust both the radio frequency amplifier 104 and the intermediate frequency amplifier 110 at the same time due to the non-obvious take-over-point (TOP), which may destabilize the radio signal receiving module 100. Therefore, providing a low cost and efficient take-over-point (TOP) AGC apparatus for a radio signal receiving module is a significant concern in the digital television industry field.